Re: [ATM] Focal Plane Location

["Bob May" <bobmay@nethere.com>]

The 1/4" distance outside of the focuser tube is a fairly good
point to start at for putting the focuser to the primary
distance.
The sad thing is that, except for a few lines of EPs, there is no
standard for the distance between the mounting stop and the focal
plane of an EP.  This is the reason whey there is such a large
focal range with focusers- they need to adapt to all of these
different distances to the scope.
Please note that the two important points on the EP are the stop
which holds the EP to a particular location on the focuser and
the focal plane of the EP.  No other distance is really important
here.  Some EP designs actually have the focal plane of the scope
buried inside the elements of the EP while others have that point
outside the elements.  Some EPs have visible field stops (thta
little ring inside the end of many EPs that you really don't
understand whey it is there) while others don't.  The field stop,
if it exists, is generallly the focal plane of the EP and you can
see it when you look through the EP as the black edge of the FOV
of the EP.
And yes, the focal plane of the scope has to be at the focal
plane of the EP ino order to see the focused image.  That is what
the EP is doing, reimaging the image that is floating there to
something that your eye can view.  The purpose of the EP is
actually to take that image and return it to parallel light that
your eye can actually see.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.

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